Filament for incandescent lamps



March 27, 1928.

G. W. MEISTER FILAMENT FOR INCANDESCENT LAMPS Filed Feb. 20. 1926INVENTOR GEORGE W ME/STA'R ;Q/%

' ATTORNEY Patented Mar. 27, 1928.

UNITED STATES PATENT OFFICE.

GEORGE W. MEISTER, OF IRVTNGTON, NEW JERSEY, ASSIGNOR TO WESTINGHOUSELAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

. FILAMENT FOR INCANDESCENT LAMPS.

Application filed February 20 1926. Serial No. 89,623.

g This invention relates to filaments for incandescent electric lampsand more particularly to a tungsten filament having a low rate ofevaporation. I

Blackcning of the envelope of incandescent lamps, is caused chiefly byvaporization of the filamentary material and volatiliza-- tion ofimpurities contained therein. such as carbon. The discoloration of thebulb due to vaporized impurities in the filament, is relatively slightand takes place mainly, during the initial burning of the filament. Thecoloration is mainly dependent upon :the vapor pressure of thefilamentary material and exists throughout the life of-the In the caseof pure molybdenumf filani ents, the rate of evaporation is so high asto preclude their use in incandescent lamps. Tungsten has the lowestrate of evaporation of those metals which are sufliciently refrac toryto withstand lamp filament temperatures. However, in the case oftungsten filaments the life of the lamp and efficiency of operation arelimited by the vaporization of the tungsten, resulting in decrease inthe candle power of the lamp due todiscoloration of the bulb.

One of the objects of the present invention is to produce a filamentcomposed chiefly of, tungsten which will have a materially lower rate ofevaporation than ordinary tungsten filaments.

Another object is to provide a tungsten filament which will have a longlife and high efficiency.

Another-object is to provide a method of lowering the vapor pressure ofa filament composed chiefly of tungsten.

Other objects and advantages will hereinafter appear. y

In accordance with this invention, the tungsten filament hasincorporated therewith a small percentage of another ingrediout whichdecreases the vapor pressure of the filament and enables it to beoperhted at a higher efliciency than in the case of ordinary tungsten orif operated-at the same temperature to give the filament a' materiallylonger life. The additive substance which I have found to beparticularly effective for producing the above result, is m0- lybdenum,when used in proportions of less than 1%. The proportion of thismaterial which I have found to produce the best result is from .05 to25%. Higher percentages while giving a lower rate of evaporation thanthat of pure tungsten, do not reduce the vapor pressure to as large anextent as does the smaller proportion.

The molybdenum in the desired proportion may be admixed in powderedstate directly with the powdered tungsten from which the wires are to befabricated or if desired, reducible compounds of molybdenum and tungstenmay be mixed in such proportions as to produce in the finished filamentthe required percentage of molybdenum. The molybdenum is employed inaddition to the usual doping of thorium oxide in the case of theproduction of thoriated filaments or in addition to the alkali metalsand other material added in the production of so-called non-sagfilaments, such as covered by the patent to Pacz, 1,410,499, grantedMarch 21, 1922, and entitled Metal and its manufacture. The mixture ofthe powdered metals is sintered and swaged in the usual manner and drawnto the desired size.

Although I do not desire to belimited to any particular theory, it isbelieved that the molybdenum in small percentages, forms with thetungsten a solid solution, the vapor pressure of which is materiallylower than the ordinary tungsten itself. When more than 1% of molybdenumis added the excess molybdenum appears to vaporize out during burning.

In order that this invention may be more fully understood, reference maybe had to tlicaccompanying drawing the single figure of which shows alamp adapted for detel-mining the rateiof evaporationot' lilaments.

This lamp comprises an envelope 1 having the filament 2 to be testedextending therethrough and connected at one end of the leading-in wire 3and at the opposite end to a spring 4 which conducts current theretofrom the leading-in wire 5. The spring l is adapted to apply a tensionof about 5 grams in order to prevent sagging of the wire. Preferably,the bulb is formed of lime glass which has high light transmissioncharacteristics. A glass slider 6 is loosely mounted in the envelope soas to be movable from one portion of the envelope to another, by tippingthe same. The function of the slider is to protect certain parts of theenvelope from deposition of the tungsten vaporized from the filament inorder to give a comparison with those portions on which the depositionis -allowed to take place. In making these tests the weight of thefilamentis carefully obtained before it is mounted in the lamp. The lampis then exhausted to a high vacuum by mercury diffusion pumps and bakedout at about 400 C. which temperature is maintained for 3 or 4 hours toremove as far as possible, all occluded and absorbed gases in theenvelope.

An auxiliary tungsten filament is then burned in the exhaust system inorder to remove water vapor from the lamp before the filament isinitially lighted. The filament to be tested is then seasoned on thepumps and the lamps sealed off. A space of about 5 millimeters is markedoff and each side of the center of the lamp into two zones 7 and 8 andthe slider located so as to cgver both zones. In this position thefilament is seasoned for about two hours at 2400 K, to insure athoroughly seasoned filament before starting the test. The glass slideris then moved so as to expose one of the zones, as 7 while covering thezone 8. The filament is then burned at a temperature of 2900 K.-* 520.At regular intervals the slider is removed from the zone 8 and readingstaken on both the clear portion of the envelope, i. e., zone 8 and onzone 7 on which the deposit has been allowed totake place. From thesereadings the temperature of the filament may be determined andtheoptieal density of the darkened portion readily ascertained.

After an optical density of from .3 to .5 is obtained corresponding to alight absorption of from 50 to 70% the lamp is broken open and thefilament removed and weighed to determine the loss-in weight. From theloss in weight the rate of evaporation may be calculated.

It was found by these tests, which were conducted with extreme care andwith filamcnts freed as far as possible from impurities which mightreadily volatilize and give inaccurate results, that the molybdenum'doped filaments showed a rate of evaporation much lower than thatobtained with ordinary tungsten, This lowering of the rate ofevaporation depended upon the amount of molybdenum in the wire andappeared to be the greatest when the percentage of mo lybdenum was below1%. Very good results were obtained using 25% of molybdenum.

WVhile molybdenum has been specified as the preferred material forobtaining the desired result, it is to be understood that the inventionis not so limited but any material may be employed which forms solidsolutions with the tungsten and lowers the vapor pressure thereof.

What is claimed is:

1. A filament for an incandescent lamp consisting chiefly of tungstenand having incorporated therewith less than 1% of molybdenum, saidfilament having a vapor pressure lower than that of ordinary tungstenfilament.

2. A filament for an incandescent lamp consisting mainly of tungsten andcontaining from .05 to 25% of molybdenum.

3. A filament for an incandescent lamp consisting of a solid solution oftungsten and molybdenum, said molybdenum being present in relativelysmall proportions whereby said filament has a lower vapor pressure thanordinary tungsten at the same temperature.

4. An incandescent lamp filament having a lowrate of evaporationcomposed mainly of tungsten and containing a. small percentage ofanother ingredient which forms a solid solution therewith.

5. A filament for an incandescent lamp composed chiefly of tungsten andcontaining less than 1% of another ingredient which decreases the vaporpressure of the filament below that of ordinary tungsten at the sametemperature.

6. The method of reducing the rate of evaporation of filaments composedchiefly of tungsten which consists in incorporating aslmall proportionof molybdenum therewit 7. The method of reducing the rate of evaporationof filaments composed chiefly of tungsten which consists inincorporating therewith less than 1% of another ingredient which forms asolid solution therewith.

In testimony whereof, I have hereunto subscribed my name this 18th dayof February, 1926.

GEORGE W. MEISTER.

